Mold coating



' a casting process,

Patented Sept. 9, 1947 MOLD COATING Walter A. Dean, Lakewood, Ohio,asaignor to Aluminum Pa",

Company of America. Pittsburgh, a corporation of Pennsylvania NoDrawing. Application November 13, 1942, Serial No. 405,483.

This invention relates to the casting of magnesium and magnesium basealloy and is directed to the problems encountered when that moltenmetal, or such alloys, is poured into contact with coatings provided onthe surfaces of a, mold cavity. While the invention has useful aspectsin connection with any mold where such coatings are desirable, it isspecifically concerned with the problem of molds of the permanent orsemi-permanent type in which at least a portion of the surfaces of themold cavity are composed of a relatively permanent material such as, forexample, a metal. Such surfaces are usually composed Of mold wall, coresand, perhaps, chills cooperating to form and define a cavity in themold.

Coatings are usually applied to the walls of a mold cavity, Prior tocasting molten metal therein, to prevent a sudden chilling of thatportion of the metal which contacts the surfaces of the mold cavity, to.prevent reaction between mold material and molten metal, to preventcavity wall erosion and sticking of the metal casting to the mold wall,and to serve other purposes, all of which are well known in the foundryart. For successful operation the coating should have the requiredthermal characteristics, should provide a surface from which a castingof smooth clean surface may be obtained, should promote ready and exactfilling of the mold cavity by the molten metal and should be resistantto the erosive or abrasive action of the incoming metal, as wellascapable of preserving its physical integrity under the varying thermalconditions to which it is necessarily subjected during repeated castingoperations. It is desirable that a balance of all of these qualities beobtained with the end in view of producing a sound satisfactory castingwithout constant expensive interruption of the'casting process to renewthe coating on the casting cavity.

- magnesium as lend to the The object of this invention is to provide amethod of coating molds which will meet the peculiar problemsencountered in the casting of magnesium and magnesium base alloy.

A further object of the invention is to provide coated molds and methodsof casting which will improve commercial casting conditions in thecasting of light metals such as aluminum and its alloys and magnesiumand its alloys.

The coatings of this invention are comprised of two generalelements-binder and insulator or body material. The coatings are appliedto the mold cavity surfaces by means of a carrier, liquid Heretofore inthe casting of magnesium and magnesium base alloys thereof the desirablequalities above mentioned have not been achieved in the mold coatingsused. Magnesium is, in molten form, a highly reactive metal, and themold coatings with which it comes in contact -ften fall after just a fewcastings are made in the mold cavity. This necessitates interruption ofthe casting procedure while the coating is renewed and this addsmaterially to the cost of particularly where a perma nent orvsemi-permanent mold is being used and costs are based upon the number ofcastings which can be produced from said mold in a given period.Moreover,

in character, other liquids may be used. The proportions orconcentration of this carrier with respect to the coating will dependupon the operators-needs and the manner in which the coating is to beapplied, for instance, by spraying, brushing or the like. The insulatoror body material may be one or more of many inorganic materials whichare used to perform this function in mold coatings. Their specificselection may depend upon several factors, including their relativechemical inertness, the temperatures involved, the thermalcharacteristics required, the nature of the desired surface of thecasings to be. produced (smooth or rough) and, with regard to obtainingoptimum coating quality and performance under particular moldingconditions, the binder employed. As is later specifically noted,colloidal graphite, furnace slag and vermiculite are particularinsulators which are adapted to achieve optimum benefits and specificresults in the practice of this invention, but other inorganic materialssuch as, for instance, other undue reaction between mold forms ofgraphite, magnesium oxide, diatomaceous silica, asbestos, soapstone,china clays, and the like, may also be used toward the obtainment of thegeneral objects above stated. Any one or more of these and otherinsulators may be used in one coating. The proportion-of insulator tobinder in the final coating is governed by considerations specific andimmediate to the which is usually water, although foundry conditionsfaced by the operator, and selection of specific proportions is governedby the intricacy and size of the mold cavity, the particular alloy beinghandled, the relative binding quality of the binder, the bulk andcovering power of the insulator, the thickness of coating desired andother well known factors. In any event, absolute proportions are amatter of simple and routine-selection. r I 1 The binder promotesadherence of the particles of the insulator or body material to the moldwall and forms a matrix which gives continuity to the coating and mayalso serve to protect the insulator particles against erosion or attackof the molten metal. Binderspreviously used for thisruption of thecasting process in order to renew the coating.

According to this invention I providea binder selected from the class ofsoluble sulphates consisting of magnesium sulphate and the alkali metalsulphates. Where, as in the case of sodium,

more than one soluble sulphate exists, I intend to include in such groupall such sulphates of that metal which are soluble in the water, orother carrier. It will be apparent that once the coating is upon thecavity surface solubility of the sulphate becomes unimportant, andindeed the action of the heat of casting upon the coating may alter thesolubility of the sulphate of the coating. Of the sulphates in the classjust described I prefer to use magnesium sulphate (MgSOc'ZHzO) but allsuch sulphates have the common property of withstanding the action ofthe molten magnesium or magnesium base alloy and of also performing theother functions desirablein a binder to produce with the insulatormaterial a relatively hard, adherent and continuous coating upon themold cavity surfaces. The practice of this inventionwill thereforecomprise the use of a coating containing at least one of these sulphatebinders, and insulator or body material. While the insulator or bodymaterial may be chosen from the wide range of materials which areavailable, I have found that certain of these cooperate with the namedsulphate binders to form coatings which are particularly superior and,therefore, I prefer to use these substances, which are black iron oxide,vermiculite, and furnace slag.

Examples of mold coatings compounded in accordance with the principlesof this invention are the following:.

Coating A Magnesium sulphate (MgSOc'II-IcO) grams 25 Iron oxide (F6304)do--.... 25

a Carrier. 100 cubic centimeters of water.

The proportions of binder and insulator above given form no part of thisinvention and will be varied by the operator to meet specificconditions. Usually I prefer to add the sulfate binder in an amount notless than about percent by weight of the total weight of binder andinsulator, butpercentages expressed by weight are notnecessarilysigniflcant since some insulators, such asasbestos,-diato'maceous silica such as Silocel, and others, have a lowweight per unit volume. Simple trial will indicate the amount of thedescribed sulphate desired. F1 The mo'idcoatings herein described may beapplieddi'rectly to the surfaces of the mold cavity.

and its alloys. The- -In some instances it may be desirable to firstwash the mold cavity surfaces with a preliminary wash that highlyreactive or'coating'to obtain a surface to which the mold coating"= willmore readily adhere. Such preliminary washes or base coatings are wellknown.

- Their-use is, of course, no part of this invention,

and since they are-completely covered by the mold coating appliedthereon, they 'do not contact the molten metal poured into the moldcavity. Other similar"-- steps such as are "normally used in foundrypractice may be undertaken in connection with the practice of thepresent invention.

While the desirable property of these sulphate bound coatings is theirrelative'permanence as compared with coatings previously used in thecasting of magnesium. neither these new coatings, nor the previouslyused coatings, are, in any relative sense, as permanent as many of themold cavity surfaces to which they are applied. In

a large modern foundries permanent mold casting is a relativelycontinuous operation in which costs depend, to a substantial extent,upon lack of interruption of the casting routine. The molds used are,through one or more production days, per manent, and a continuous supplyof molten metal to such molds is merely a matter of arrangement. Givengood foundry conditions the casting routine can therefore proceeduninterrupted provided the coating on the mold cavity does not fail andthus necessitate interruption of foundry procedure in order that it maybe renewed.

Another aspect of my invention is based on my discovery that in thecasting of light metals,

such as aluminum and its alloys and magnesium and its alloys, thelongevity of any mold coating may be increased if there is provided onthe mold coating a superficial layer selected from the class consistingof soluble magnesium sulphate and soluble alkali metal sulphates. Sucha, layer, applied in any convenient manner as by spraying in solutionforms a hard layer on the coating which adds to the effective life ofthe mold coating without materially altering the functionalcharacteristics thereof. This layer prevents severe erosion, minimizesreaction and generally increases the longevity of the coating to whichit is applied. In applying such a superficial layer, neither the mannerof application nor the concentration of sulphate compound is a criticalfactor. The concentration employed will govern the applicationsnecessary to produce a superficial layer of a given thickness. I haveused, with good results, a solution containing 25 per cent .by weight ofmagnesium sulphate (MgSO4.7H2O) and applied this solution by spraying.The desirable result is a well-dried continuous superficial hard layerof magnesium sulphate or alkali metal sulphate imposed on the moldcoating.

, necessary to bind the insulator to the mold and to produce the coatingcontinuity Where the mold cavity is,'at least in part, defined by anon-permanent material, such as a sand core, no problem of permanencearises since the cavity. is, to that exent, destroyed after each castingoperation. However, the value of the superficial layer in decreasing thereaction of the molten metal on the core coating and in preventingerosion during casting is sufficiently great as to justify its use inmany cases.

While certain preferred embodiments of the invention, and specificexamples thereof, have been described, it will be understood that suchare by way of example only and that the invention may be otherwisepracticed within the scope of the appended claims. It will also beunderstood that the mold cavity "surfaces" referred to in the appendedclaims may be originally bare or originally p ovided with a preliminaryor base coating.

I claim:

1. In the method of casting light metal into 20 mold cavities at least aportion of the surfaces of which are provided with a coating containinga binder and inorganic insulator material, the improvement consisting infacing the coating with a layer of a compound consisting of magnesiumsulphate and alkali metal sulphate.

2. In the method of mold cavities at least which casting light metalinto a portion of the surfaces of are provided with a coating containingselected from the class.

a binder and inorganic insulator material, the improvement consisting infacing the coating with a layer of magnesium sulphate.

3. In the method of casting magnesium and magnesium base alloy in moldshaving mold cavity surfaces coated with a coating containing a binderand inorganic insulator material, the improvement consisting in formingthe coating with a binder which is composed of a substance selected fromthe class consisting of magnesium sulphate and alkali metal sulphate andin facing said coating with a layer of a substance selected from saidclass. 1 4

WALTER A. DEAN.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITE!) STATES PATENTS Certificate of Correction Patent No. 2,426,987.September 9, 1947.

WALTER A. DEAN I It is hereby certified that the error appears in theprinted specification of the above numbered patent requiring correctionas follows: Column 2, line 39, for casings read casting; and that thesaid Letters Patent should be read with this correction therem that thesame may conform to the record of the case in the Patent Ofiice.

Signed and sealed this 25th day of November, A. D. 1947.

THOMAS F. MURPHY,

Assistant Commissioner of Patents.

